1. Field of the Invention
This invention relates to a shaped article comprising a thermoformed composite sheet having an outer surface layer of thermoplastic polymer attached to a surface of a conformable fabric layer. In particular, the invention concerns such a shaped article in which the other surface of the fabric layer of the composite sheet is attached to a layer of rigid foam.
2. Description of the Prior Art
Conventional thermoforming processes are described by Lubin, "Handbook of Composites", Van Nostrand Reinhold Company, 1982, pages 366-367. Such processes generally involve sequential steps of (a) heating a thermoplastic sheet until it softens, (b) forming the softened sheet in a mold and (c) allowing the formed sheet to cool and harden. When thermoforming is used to mold a thermoplastic sheet into a large article, such as the interior surface of a bathtub, sink, canoe hull, or the like, the sheet undergoes a large increase in area, often by a factor of three or more, along with a corresponding decrease in thickness. However, during thermoforming, regions of the thermoplastic sheet that are subjected to large changes in area and/or contour (e.g., in corners and edges of the shaped article) often suffer from excessive and nonuniform thinning, which in turn leads to lower impact resistance and greater fragility. To compensate for such undesired thinning thinning, extra reinforcement to such regions is provided by using a much thicker starting thermoplastic sheet for the thermoforming. This results in a heavier and more expensive molded article.
Composite sheets comprising thermoplastic sheets laminated to a fabric layer are known for use in thermoforming processes. Although such materials may have been useful in thermoforming, improvements in the formed product are desired, particularly in minimizing thinning of the formed walls and in increasing the impact resistance of the exposed plastic surface.
To alleviate the wall-thinning problems associated with thermoforming of large articles, the present inventor found that considerable improvements in wall uniformity can be achieved by using a thermoformable composite sheet having a layer of thermoplastic polymer adhered to a relatively light, conformable, nonwoven fibrous layer stitchbonded with an elastic thread, which fabric amounts to no more than 20% of the total weight of the composite sheet. Although these composite sheets reduce undesired thinning during thermoforming, the utility of shaped articles formed therewith would be further enhanced if the articles also had improved resistance to cracking under impact. Accordingly, it is an object of this invention to provide increased impact resistance to such thermoformed shaped articles.